1. Field of the Invention
The present invention generally relates to semiconductor devices, and particularly relates to a semiconductor device which is provided with an output circuit capable of adjusting a signal drive power.
2. Description of the Related Art
As signal frequency is raised for the purpose of increasing transmission speed between semiconductor device chips, signal overshoot, undershoot, ringing, reflection, etc., emerge as factors that prevent high-speed and reliable data transfer. In order to reduce the effects of these factors, it is necessary to perform impedance matching by matching the transmission load with the signal drive power for transmitting signals from the semiconductor device.
It is difficult, however, to construct a system by designing transmission lines such as to make them match the signal drive power of the semiconductor device. When a system is constructed by using semiconductor devices, the load of the transmission lines vary depending on the configuration of the system. In order to achieve high-speed and reliable data transmission, however, there is a need to decrease the signal drive power in light of the load of the transmission lines if the load is small, and there is a need to increase the signal drive power in light of the load of the transmission lines if the load is large. It is thus preferable to provide a semiconductor device with the function to adjust the signal drive power according to the transmission load.
Patent Documents 1 through 3 disclose configurations in which a semiconductor device is provided with the function to adjust the signal drive power. In Patent Document 1, an output buffer is provided with a selector serving as an output current control circuit, which is controlled by a control signal supplied from an exterior, so that the output current is adjusted to a value suitably adapted to the system. In Patent Document 2, a plurality of transistors having different sizes are connected in parallel to form an output buffer circuit, and the on/off state of each transistor is controlled to adjust the impedance of the output buffer circuit. In Patent Document 3, first and second current drive units contained in an output buffer are selectively set to an activated state or deactivated state at a process following the manufacturing process, thereby adjusting the current drive power of the output buffer.
[Patent Document 1] Japanese Patent Application Publication No. 5-206829
[Patent Document 2] Japanese Patent Application Publication No. 2002-111474
[Patent Document 3] Japanese Patent Application Publication No. 2002-300023
In semiconductor devices, it is unavoidable that there is variation in the operating characteristics depending on the process conditions of the manufacturing process. As a result, the signal drive power of semiconductor devices may differ by a factor of 2 among chips. In such a case, even if the function to adjust the signal drive power is provided in the semiconductor device, such function may not be sufficiently effective.
A system may be configured such that the signal drive power can be selected from 25%, 50%, 75%, and 100%, for example. A power of 50% may then be selected in light of the transmission load. If the signal drive power varies by a factor of 2 between the chips, setting the signal drive power to 50% in a given semiconductor chip may result in the actual signal drive power being as large as 100% or as small as 25%. Accordingly, even if the system controller makes a proper selection suitable for the transmission load, an expected signal drive power may not be attained. Because of this, a need arises to design a system by taking into account variation in the signal drive power, which functions as a factor to place an upper limit to the operating frequency.
Accordingly, there is a need for a semiconductor device which can achieve a desired signal drive power without being affected by manufacturing variation.